1. Field of the Invention
The invention relates to insulation for use in liquid filled transformers.
2. Description of the Related Art
Since the invention of the first transformers in the 1880's, cellulose paper has been used as the primary solid electrical sheet insulation in liquid-filled transformers. Cellulose has several shortcomings such as moisture absorption, water generation, and limited thermal capabilities. To overcome moisture absorption, the transformer must undergo a time consuming heat and vacuum process to remove the absorbed water, so that maximum dielectric strength can be obtained. Once the heat and vacuum process has been undertaken, the cellulose is typically impregnated with mineral oil to slow the absorption of moisture. The second problem, water generation, naturally occurs as cellulose ages due to heat. This results in reduced dielectric strength of the oil, and may eventually cause a transformer to fail. The major cause of transformer failures is the limited thermal capability of the cellulose insulation. Aging of the cellulose in liquid-filled transformers results in mechanical embrittlement and electrical deterioration of the insulation. For the reasons stated, there is a desire to extend the life of liquid-filled transformers.
An alternative to the cellulose insulation is the use of enamel insulation. This insulation employs an enamel coated wire, as opposed to paper insulated wire. The enamel types of insulation are sensitive to the cleaning of the substrate (either aluminum or copper) and may have pinholes or non-uniform build across the surface of the conductor that will lead to reduced dielectric performance.
Another type of insulation that has been studied recently is aramid paper. However, this type of insulation is considerably more costly than the standard cellulose type. Also, the aramid papers require a longer processing time to reach an acceptable moisture level.
prior art does not provide for a cost effective alternate electrical sheet insulation material that has better heat aged electrical properties than cellulose.